Timely Post Herbicide Application Protects Corn Yield

Corn growers know how to control tough weeds. Yet, research shows if farmers focused more on when to control weeds, they may harvest more corn in the fall.

According to Iowa State University studies, weed emergence in Iowa fields typically begins in early April, peaks in early May and then tapers off in late May and early June. That means in most years farmers will see tough weeds such as giant foxtail, giant ragweed and velvetleaf break through the soil at the same time as their corn. And in a contest to capture available nutrients, moisture and sunlight, a young corn crop is usually no match for even small grass and broadleaf weeds.

ISU Extension weed management specialist Bob Hartzler says, "When weeds emerge with the crop and are present at moderate-to-high densities, delaying herbicide application until weeds reach a height of 4 inches or taller may result in significant yield losses. Yield losses accumulate rapidly once the critical period is reached, and losses up to a bushel per acre for each day application is delayed may occur." He defines "high densities" as more than five weeds per square foot.

Even small weeds can steal corn yieldHartzler says that weeds even as small as 2 inches can affect corn yields if present at high densities. That is, more than 10 weeds per square foot.

Plus, some weeds are just naturally more competitive with corn. The following table demonstrates the density of numerous weed species estimated to cause a 5% yield loss in corn, based on 160 bushels per acre yield potential. The value of a 5% crop loss is equivalent to the cost of some postemergence weed control strategies and can be used by farmers as a rough economic threshold.

Weed density required to cause a 5% yield loss in corn

Weed Species

Number of weeds per 40 ft . of row

Cocklebur

7

Common ragweed

26

Foxtail

39

Giant ragweed

10

Lambsquarters

26

Pigweed

15

Sunflower

4

Because timing is so critical, most weed scientists recommend farmers use a preemergence herbicide to address weed control early and provide some residual, which can help widen the window of application timing for post herbicides.

However, ISU weed scientists report between 15% and 20% of Iowa cornfields were planted in 2006 without any preemergence herbicides, making timely post applications even more critical. They recommend farmers scout cornfields within two weeks of planting and make herbicide applications earlier and not later.

Hartzler says a conservative recommendation would be to make initial postemergence applications before weeds reach 2 to 4 inches in height.

Good reasons to rotate herbicidesAndy Hurst, product manager for Liberty herbicide, a nonselective postemergence herbicide made by Bayer Crop Science, agrees. The company recommends treating small weeds with post products to maximize control and protect yield potential. "We know that corn growers value the application freedom offered by a nonselective herbicide system," he adds. "But treating tall weeds is counterproductive. The field may look good, but its yield potential is already negatively impacted."

Hurst says delayed herbicide applications and overdependence on a single chemistry have contributed to weed management problems in recent years. Farmers who use glyphosate in the same field continuously year after year run the risk of weeds developing resistance to glyphosate.

For those reasons, he encourages farmers to rotate active ingredients in their weed control program. Or if a farmer prefers to stay with a nonselective herbicide system, then use Liberty as the option. Liberty offers the same ease and convenience as glyphosate. Plus, with its unique mode of action, Liberty gives farmers a method for effectively managing weeds and potential resistance issues. "If farmers will rotate their herbicide program from year to year and focus on treating weeds when the weeds are small, we will be able to keep these non-selective herbicide systems viable for years to come," he says.